In U.S. Pat. No. 4,091,683 issued May 10, 1987, a system for downhole measurement of pressure is disclosed in which an interrelated capacitance device can be used in a transducer for measuring pressure. In U.S. Pat. No. 4,322,775 issued Mar. 30, 1982, a sensor or transducer construction is disclosed in which the measurements of two capacitors in a sensor construction are obtained and where there is a compensation for the effects of gravity and temperature. In U.S. Pat. No. 4,624,136 issued on Nov. 25, 1986, a typical downhole oil field tool for measuring pressures and temperatures is disclosed.
While the present invention has a wider application, one of the critical areas of concern to the evaluation of oil field reservoirs involves the determination of downhole pressure and temperature in a well bore. For this reason, pressure and temperature measurements are taken during actual drilling operations in procedures called "drill stem tests". Also, after a well is completed it is a common occurrence to measure the downhole pressure and temperature over a period of time for "production tests". In oil field applications, therefore, measurement of pressure and temperature is an important factor and is made even more important in that the accuracy of the pressure measurement can be a significant factor in determining the extent of oil reserves available from underground production. The extent of oil reserves is an asset and thus precise pressure measurements are necessary for accurate asset evaluations. Precise pressure measurements, in turn, are a function of the transducer reliability, repeatability and accuracy.
Oil field practices, being what they are, cause equipment containing one or more pressure transducers to be subjected to rough treatment which translates into shock loading for transducers and other instrumentation in a well tool. It is not uncommon for a tool to be dropped at the earth's surface, in a shop, at a well site or transported at a haphazard speed into or out of the well bore. In some operations it is sometimes possible to have the pressure and temperature tool connected in a system where a shaped charged perforator or other explosive device is detonated in the well bore. Thus, the pressure and temperature measuring tool can be subject to moderate to severe shock loads in a well bore. Preferably, the transducers and instrumentation should have a construction which minimizes the effects of shock loading.
Another significant factor in pressure and temperature measurements is the fact that a temperature change can occur while a tool is in a downhole well situation. Because there is a temperature response lag factor involved in equalizing temperature, the pressure recording obtained while a transducer is equalizing temperature can be inaccurate until the transducer stabilizes at the changed temperature. Thus, there can be a time span of inaccuracy in the pressure measurements while a temperature stabilization process occurs.
In a typical oil field production test of temperature and pressure, the tool is lowered typically on an electrical wire line, slick line or tubing into the well bore and the tool is used to sense pressure and temperature data over a period of time. This data as collected over the period of time is either read out at the surface or retained in a downhole memory for subsequent analysis of pressure as a function of time and of temperature as a function of time.
In a non-oil field application a transducer can be utilized for measuring force, i.e. a weight or force applied to the transducer can be measured with a high degree of accuracy. The transducer can be used for measuring pressure which produces a corresponding force.
Quartz constructed capacitance measuring devices for measuring force or pressure have been used in a number of surface applications. Typically, such applications do not parallel downhole oil well pressure and temperature conditions. Also, size and shock mounting of a transducer is not usually a factor in the design for a surface application. Finally, the accuracy of measurement by surface type of quartz tranducers, while several order of magnitude better than strain gauges, still lacks the precision desired for many applications. This is particularly true for accuracy desired for downhole temperature and pressure measurements in a well bore.
Capacitance pressure transducers utilizing quartz and metal films can be found in
U.S. Pat. No. 3,952,534 (Birchall):CL 317/246
U.S. Pat. No. 4,388,688 (Bell):CL 361/283
U.S. Pat. No. 4,196,632 (Sikorra):CL 73/718
Glass frit for attachment of quartz members can be found in
U.S. Pat. No. 4,168,518 (Lee):CL 361/283
Force responsive transducers can be found in
U.S. Pat. No. 4,295,376 (Bell):CL 73/724
U.S. Pat. No. 4,382,385 (Paros):CL 73/702
U.S. Pat. No. 4,507,972 (Morita):CL 73/708